In preparing rubber stocks for use they are generally mixed in a Banbury with some or all of the compounding ingredients, and the rubber stock is sheeted on a sheetoff mill and cooled for further use. If it is to be stacked or stored before further handling, it must be dusted or coated with a material like clay to prevent sticking. The output of a Banbury for a particular rubber compound has generally been measured by the time or by the temperature required. It has been found that measurement of the energy consumed by the Banbury by means of a power integrator (Monsanto) gives a better indication of mixing. Mixing of the rubber compound or mix in the Banbury usually results in a large heat build-up with the temperature of the compound increasing to about 90.degree. C., and higher most of the time, even up to at least about 150.degree. C.
Slab dips are used in the tire industry for coating freshly mixed rubber compounds to prevent them from sticking together when piled and shelved for subsequent processing. The installation of power integrators on Banburys significantly reduces the mixing time of rubber stocks, and hence it increases throughput, increases savings or reduces the need for more Banburys or mixing capacity. However, the method of cooling the rubber compounds by forced air (fans) is not adequate generally to accommodate the increased throughput for a given factory space. It requires more than 12 minutes to cool a stock from the Banbury and sheetoff mill with forced air to a desired laydown temperature of about 50.degree. C. maximum, preferably of about 45.degree. C. maximum, to prevent scorch. A cooling system incorporating water spray units between the slab dip tank and the air cooling fans effectively reduces the cooling time to less than 5 minutes. The use of water sprays, however, renders the usual slab dip, a clay dispersion in water, ineffective as it is easily washed away when sprayed with water.
Slab dips usually come in the form of an aqueous suspension or emulsion containing suitable inorganic or organic substances. Most commonly used additives include whiting, clay and/or paraffin wax. The effectiveness of the slab dip depends on the uniformity of the coating deposited on the rubber surface, particularly on drying, and on its resistance to flow or displacement under stress as a result of stacking. Many compositions probably would work if they were dried by air or forced air after deposition on the rubber surface. The requirement of resistance to removal by water spray would limit the number of usable candidates. Zinc stearate slurry in water can be used as a coating. When deposited on the rubber surface, the zinc stearate forms a uniform, anti-sticking film which resists removal by water. However, the cost of zinc stearate even in thin coatings outweights the benefits resulting from the use of a power integrator on a Banbury.
Accordingly, it is an object of the present invention to overcome the difficulties alluded to hereinabove and to provide a method for coating a hot rubber compound with a coating which resists removal by water, which enables the rubber to be cooled substantially rapidly by water and which enables the cooled rubber to be assembled or stacked without sticking.
These and other objects and advantages of the present invention will become more apparent to those skilled in the art from the following detailed description and working examples.